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Why does the P-51 Mustang take so much altitude to recover from a spin?
From this answer, it appears that the P-51 Mustang takes a lot of altitude to recover from a spin. The average is a 3000 to 6000 foot loss, with a loss of up to 7500 feet in one instance.
The recovery also seems lengthy. From the document quoted in the answer:

The average spin [...] requiring 3-1/2 turns for recovery.

What makes the P-51 Mustang so difficult to recover from a spin?

Possible duplicate of Does it really take 9000 feet to recover from a spin in a P-51 Mustang?

@gusto2 not really. that is about "how much" this is about "why"

Others can probably give you more precise info since I'm not a P-51 pilot, but I am studying aircraft dynamics in school right now, so maybe this will help.
Many fighter aircraft are designed such that they are very agile. This is especially true with roll dynamics, where modern fighters are even designed to be inherently unstable. They can then roll faster since the dynamics promote the roll. For roll stability, the wing design is often the main concern. Now, to spins.
I believe spins are mainly a yaw-plane event (though I'm sure there is some coupling of the equations). this means its a rapid rotation about the z-axis, the axis normally considered down. For stability, or desirable spin recoverability characteristics, the tail design is key. I don't know all the details or considerations behind the P-51's tail design, but any number of things could cause slow spin recovery, meaning you need to fall for a while (sorry, pilot!)
For example consider the distance between the tail's centroid and the overall plane's center of gravity. Imagine we added a lot more fuselage, basically extending the tail farther out. Now, with our extended tail, we have a different system. a really long tail might be too resistant to any yaw - basically it wants to got a certain direction, like an arrow.
Now, shorten the body instead, making like a short little squished P-51 with the tail right behind the pilot(shorten the arrow). while flying, the tail now might not help the plane fly very straight, at least not easily.
Finally, put the tail in front of the pilot and try to find anyone brave enough to fly it! No one will want to, because they won't make it home. its like shooting an arrow backwards, or throwing a paper airplane backwards- the dynamics of the vehicle just aren't favorable - any sort of deviation and the instability induces more deviation.
Thats only one part of tail design. You also need to consider size, number of tails, control surface size, and this doesn't even include the yaw dynamics that come from the rest of the plane in a spin.
I apologize if you already understood all that, but maybe it would help with my key idea. That is, that the reason it takes so long to recover from a spin is due mainly to its tail design. I don't know specifically how they designed it, or what design goals they were looking for, but because of what they chose, maybe considering manufacturing ease, structural considerations, aesthetics, drag, turn performance, weight, heck, maybe they just sketched it up and tried it, they ended up with a plane with this yaw plane recovery performance. It might be bad, but maybe their other design goals were met and they chose to live with it.
Other aircraft with better spin recovery performance will likely have different tail features like larger control surfaces, different tail volume... Overall, their tail design will give it better yaw stability and authority (control power available) to recover sooner.
Its late, and I'm sure I've missed things. I hope others can chime in and fix anything I may have mistaken. Let me know what I can add to improve my answer

3 1/2 turns seems pretty good, but factors to consider are:
The P51 Mustang has a similar wing area compared to a Cessna 172, but weighs 6x more.
The Mustang was designed for as low a drag as possible.  Great for long range fuel economy and terrific for chasing someone in a dive unless ... the ground happens to get in the way.  It also has a somewhat finicky laminar flow wing that could easily re-stall if recovery was not done right.
Then add our natural human tendency to skew distance and height perception (from when a 30 foot fall from a tree was "bad").  9000 feet is waaaaay up, but 2 miles on a track is no time flat at 200 mph.
Massive and drag slick airliners have similar issues "pulling out" of dives, with rapidly increasing speed even with power at idle.  Not much time to recover.
So take a 172 spin and recovery of 1500 feet, and multiply in these factors, and 6000 to
9000 feet is very believable, for those lucky enough to tell the tale.

Weight and airfoil.  That 6x wing loading factor is the big one...

Comments on tail design are relevant here. Much of the documentation quoted seems to refer to A,B and C model 'stangs for which the tail was basic. From D model and onward, a dorsal fin was added between vertical fin and fuselage to aid in stability and control. Also this is a combat aircraft of some significant mass and wing loading and this may contribute to greater altitude loss than a recreational GA aircraft in a given recovery procedure.

I stand corrected - dorsal it is.

One reason given for adding the dorsal fin was that the A,B,C models had a canopy that extended back to the tail which provided lateral stability. The canopy on the D model was a bubble canopy and needed the dorsal fin to restore the stability lost.

The POH for the P-51 (p. 24) indicates that stalls and spins are easily handled. However, the war stories I have read and videos I have seen indicate that a spin in a P-51 could be quite violent. This often happened during dogfights where the pilot was pulling hard Gs in a turn at full throttle. (See, e.g., this article which notes that "normally understated" test pilots referred to the stall as "viscious".
From what I have read, the reason for this behavior was that the P-51 had "thin" laminar flow wings which can stall suddenly and without warning. (See, e.g., Bradley C Hood, "Fighter Formation Fundamentals" (available online) ["Due to the P-51’s laminar flow wing. It exhibits accelerated stall characteristics much like that of a modern jet fighter."]) In contrast, aircraft with "fatter" wings, like the Cessna 172, have fairly docile stall characteristics.
While a stall is merely the entry into a spin, the wing characteristics that got you into the spin can make it equally difficult to exit the spin.  (By "thin" and "fat", I am referring to the shape of the wing cross-section, not the actual dimensions.) (See Anatomy of airfoils and their performance, p. 38 for a comparison of the behavior of thin vs. fat wings.)
(Working at Cessna, I was told that the Cessna Cardinal - which was intended to replace the Cessna 172 - had a similar problem. It had laminar flow wings which were not as forgiving as the fatter Cessna 172 wings, leading to some rough landings.)
As to why the particular spins mentioned elsewhere were especially bad, here are some additional possibilities:

The POH indicates that there is a difference between left and right spins.
The aircraft could have had almost full fuselage tanks which created an aft CG. The POH (p. 22) indicates that the aircraft was very unstable with full fuselage tanks.


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What aircraft fly faster than 250 knots indicated airspeed under 10,000 feet for safety reasons?
14 CFR 91.117 limits aircraft to 250 knots indicated airspeed below 10,000 feet, but 91.117(d) allows aircraft to fly a higher speed if the minimum safe speed is higher. What aircraft and aircraft conditions might require an indicated airspeed higher than 250 knots?

If the bleed air system isn't working, the F-104 has a landing speed of 240kts since it uses a blown-flap system. It may be reasonable for a pilot to be above this speed on approach. Normally I wouldn't suggest a military aircraft, but there are privately owned F-104's. Some aircraft like the 747 get pretty squirrely when clean and heavy below those speeds, I think the A380 may be in the same boat.

The limit is 250 knots indicated air speed (KIAS). IAS is not corrected for instrument and position error nor is it corrected for winds. Thus any aircraft can exceed 250 knots ground speed as long as their air speed indicator shows less than 250 KIAS. This can easily happen with tail wind component.

@SteveKuo I wasn't confused about the type of airspeed, but I'll update the question to clarify.

I'm also interested in a definitive list.

Possible supplemental question-- in what US airspace below 10,000' MSL would 14 CFR 91.117 generally not apply to military aircraft?  Also included in the scope of the question should be in what types of airspace has the Administrator made a permanent blanket authorization to exempt  military aircraft from 14 CFR 91.117. (Thinking of MOAs, restricted airspace, low-level training routes, etc.)   Feel free anyone to grab it, tweak it,  and ask it unless the answer is too obvious.

Actually it appears the answer to my proposed question is given in the letter attached to the accepted answer below.

FAA has letters of agreement with US DOD, which authorizes certain military aircraft to fly faster than 250 KIAS below 10,000 feet. A good example of this would be the Northrop T-38 Talon, described in FAA JO 7610.4 Special Military Operations:

12-10-11. AUTHORIZATIONS
T-38/T-1 airspeeds in excess of 250 KIAS below 10,000 feet MSL are authorized by competent military authority in accordance with FAA Order 7110.65 and the letter of authorization granted to DOD.

The reason for this waiver is discussed in NTSB report number FTW90FA151:

THE T-38'S SPD WAS ABOUT 330 KTS; THE SPD LIMITATION OF 250 KTS HAD BEEN WAIVED FOR T-38 ACFT, DUE TO THE ACFT'S LACK OF CONTROLLABILITY AT THE LOWER SPD.

The relevant letter from forums.jetcareers.com:

Also, from AIR FORCE INSTRUCTION 13-201:

14 CFR Part 91.117 covers aircraft speed. Recognizing that some DoD aircraft performance requirements exceed 250 knots, the FAA issued an exemption to 14 CFR Part 91.117.

The exemption has been claimed for other aircraft too- like F-15 Eagle, for example:

... climb-out occurs at 350 KCAS for an air-to-air configured jet and 330 KCAS on one with air-to-ground ordnance. ... this is significantly higher than the 14 CFR speed restriction of 250 knots below 10,000 feet. The F-15E, like most fighter aircraft, falls under the Letter of Agreement between the DoD and FAA allowing some military aircraft a waiver to that speed limit.

When the Concorde was in operation, its takeoff speed was about 220 knots. The delta wings on the Concorde and most other supersonic military aircraft require a high angle of attack and higher takeoff and landing speeds. Futhermore, the Concorde used afterburners on takeoff, so it was accelerating. That means that the Concorde probably also flew well over that speed limit below 10,000 ft.

Ye thirde linke breaketh.

A 747 taking off at or near max gross weight will have a flaps up, minimum safe airspeed and climbout airspeed speed greater than 250 knots. In such cases you didn't notify ATC as they expected it.
I tried to find what the exact numbers would be at 840,000 lbs, but I seem to have thrown out that manual. I did, however, find a reference for a 747-200 with JT9D-7Q engines that gives the flaps up holding speed at 800,000 lbs as 259 kts at 5,000 feet.
Also, I do remember being on an arrival once, with the weight low enough that we could safely maintain well below 250, but that when we saw we were icing badly, we speeded up to somewhere around 300 to get the ram air temp above freezing. In that case we notified ATC that we were doing so.
Also, ATC occasionally requests max speed. For example, on Hajj flights into and out of Jakarta Halim airport in the late 1990s it was common practice for controllers to put you down low to get you under the traffic into Cengkareng airport. They would then often request max speed.

@ymb1 Correct, but there is one small caveat in that while most 747 carriers chose to go up through the flap settings from takeoff to a clean wing in 20 knot increments because that made it easy for the pilots to set the bugs, there was at least one carrier, the first one I flew for, that insisted the exact speeds be used. A plastic placard plastic placard grease-pencilled by the f.e. and checked by both pilots for the takeoff weight that gave the exact speed for each flap setting was put on the thrust lever console. Thus V2+ in that system was a few knots different for each flap setting.

A number of military aircraft - one that I can cite off the top of my head is the T-38 - have speed restriction waivers from the FAA during operations for safety.  The T-38,  for example, flies the traffic pattern at 270 KIAS in order to provide sufficient maneuvering capability without exceeding the AoA limitations of the aircraft and stalling it.  The F-104 probably has similar limitations in performance.  The SR-71 rotated at 230 KIAS and the pilot had to immediately retract the gear after liftoff to prevent structural damage from exceeding Vle of 300 KIAS.
As for commercial and civil aircraft, the speed restriction of 250 KIAS does not present much of an obstacle or impediment to safe operation, albeit it’s a little slow for jets.

777-200 stall speed at max t.o. weight is 192 kts. 1.3 buffer brings that to 252 kts.
777-300 stall speed at max t.o. weight is 207 kts. 1.3 buffer brings that to 269 kts.
Because of the higher stall speeds, especially for the 777-300, ATC knows we'll exceed 250 kts on departure and frequently acknowledges it before we mention it. On arrival, at lighter weights, it's not an issue. On the 777-300 you don't have to have Flaps 1 out until approx 220 kts. at max landing weight. On the 777-200 it's slightly lower (approx. 215 kts). So the reduction in weight from max T.O. weight to max landing weight reduces the minimum clean speed by 35 kts (777-200) and 50 kts (777-300).
Most fighters, in normal ops with a clean wing, won't be close to stalling at 250 kts. But they can pull more G's, thereby having more maneuverability, if they're flying faster.

Nice info, and welcome to Av.SE!  Would it be correct to assume that the 192 and 207 knot speeds are in a clean configuration?

In the EA-6B we had a waiver to climb out at 300KIAS.
Coming home we were lighter and could easily make 250, but we were waivered to 300 if we were in formation to have extra margin for the wingman.

At Rota or Atsugi?

I was at Whidbey

Why would you have to fly faster with a wingman?

Better responsiveness from engines and flight controls.  The plane gets a little mushy with flaps up between 200 & 250.

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Has a fixed-wing aircraft ever been built that featured multiple turbines powering a single propeller?
Has a fixed-wing aircraft ever been conceived, prototyped or built that features multiple turbines powering a single propeller? The concept of a twin-engine aircraft with a single propeller is intriguing to me. Picturing something like a Pilatus PC-12 with two engines under the hood and a single prop might have some advantages. I come from a fixed-wing background and know little about helicopters, but if something exists I suspect it is a rotor wing.

Aircraft, yes, as in helicopter. Airplane I'm not so sure. The Osprey can power both propellers with a single engine (the other way around). The only one I know of that actually does this with a PT-6 is the Soloy Dual-Pac

Wow I have to look into how the Osprey does that. That's nuts.

The obvious reliability problem with this is "what happens after a single engine shutdown". With a conventional twin aircraft, the answer is "usually, nothing very exciting". At least a helicopter can do an autorotation landing, unlike a plane. If you have to add a lot of complexity (and weight) to handle that scenario, the advantages may evaporate.

There would also be the regulatory issue of whether this counts as "single engine operation over water," which in general is only permitted if the plane can reach land at a location suitable for a forced landing.

You you get most of the disadvantages of managing multiple engines while still having 1 common gearbox which is a single point of failure. So it's likely that certification authorities will object to treating it as an actually twin-engine aircraft.

Do you mean propeller or rotor?  There's a non trivial difference.

@RyanBurnette A simple analogy is "how does a differential work for a car" but the Osprey is a novel solution.

@RyanBurnette  I think you should specify whether or not you mean fixed wing aircraft, or are including rotary wing as far as aircraft go.

@alephzero Actually, with a twin engine helicopter losing one does not necessitate an autorotation in most cases.  (Examples I am familiar with are Huey, Blackhawk, Sea Sprite).  But one may need to do a run on landing versus hover landing depending on a lot of factors of only one of the two engines is running.  (Have not flown the S-76 but I heard that single engine in that helicopter was a bit more of a challenge).

@KorvinStarmast Canadair CL-84 did most of that  in 1965, so it's hardly novel.

I meant to use the word propeller, not rotor, and the question was about fixed-wing aircraft only. Made both edits.

The LearAvia LearFan 2100 used two separate PT-6B to drive a single pusher propeller through a common gearbox.

LearFan 2100 in flight (picture source)
If you want to know why its official first flight date is December 32, 1980, read here.

LearFan 2100 engine arrangement (picture source)
This engine arrangement was the LearFan's eventual undoing when the FAA denied it a proper certification because of concerns that the single gearbox constituted a single point of failure.

... doesn't the propeller also constitute a single point of failure, then?

@immibis: Haha, good point. Yes, it does, but the FAA had more concerns with the gearbox. Propeller failure outside of ground contact is very rare; gearboxes, however, are a different story, especially when poorly maintained.

From the source (article in Popular Mechanics) it seems more that with few test flights they had already demonstrated excessive wear on the gearbox. FAA denied certification because they've seen it as signs of imminent failure, the single point made it only slightly worse.

"If you want to know why its official first flight date is December 32, 1980, read here.". "Here"'s link is dead, please fix if possible!

@WilliamR.Ebenezer Thanks for letting me know. All I could do was to replace the dead link with another one which hopefully will last longer.

Well multi engine helicopters like the Bell 430 or the Westland EH101 do it all the time.
I don't recall any current production fixed wing aircraft using multiple engines to drive a single propeller, but the ill fated LearAvia  LearFan used this propulsion arrangement, using two PT-6 engines to drive a gearbox connected to a pusher prop.

Yes, see Soloy's Dual-Pac concept.

Image courtesy of Soloy
The Soloy Pathfinder 21 is powered by the Dual Pac PT6D-114A. It is essentially an extended and redesigned Cessna 208 with exactly what you describe: two PT6 engines geared to one prop hub. I understand that the project has not progressed beyond the prototype built.

Image courtesy of Soloy
The Dual-Pac was also tested on a DeHaviland DHC3 Otter platform:

Image courtesy of Soloy
Quoting from the Dual-Pac page:

Soloy Dual Pac – Twin Engine reliability with single propeller symmetry. The Dual-Pac was designed, patented and Certified by Soloy to safely combine the output of two independently operating Pratt Whitney PT6D-114A engines for a single propeller output. The system was developed with extreme redundancy so that single engine operation is not only safe, but able to be done intentionally under certain conditions.

The Ayres LM200 Loadmaster was a similar design concept, though a clean sheet design using a different powerplant and also ultimately unsuccessful for economic reasons.
See also the rotorcraft applications of the P&WC PT6T Twin-Pac, such as those that Carlo Felicione mentions.

Well, there is the Armstrong Siddeley Double Mamba engine.
It powered the Fairey Gannet
It wasn't a true single prop, but rather a contra-rotating prop (which amaze me).
Found a few more, some being single contra-rotating, others......multi-multi-engine?
Aircraft powered by the Allison T40
There is one example of the twin turbine to single shaft Allison T40 powering a single propeller aircraft...the Republic XF-84H
The thing was apparently as loud as the Tu-95, due to having a prop that spun at supersonic speeds on its outer edges.  And the prop created a continuous visible shockwave..  Personally, I am not sure if you would classify this as an aircraft, or more of a shock/awe weapon.

Well, the shockwave was much more audible than visible. Audible enough that people standing nearby got literally sick.

@PeterKämpf And strong enough to knock people over if they weren't careful.

Don't you mean a shockwave / awe weapon?

Interesting tidbit re T-40 --"The resulting T40 combined two Allison T38-A-1 power sections side-by-side with a common reduction gearbox powering contra-rotating propellers. Similar in layout to the Armstrong Siddeley Double Mamba, the T40 differed in that each engine drove both the forward and the rear propellers, unlike the Double Mamba, where each Mamba power section drove either the front or the rear propeller separately.[1]"  Wikipedia on T-40 quoting "Kay, Anthony L. (2007). Turbojet History and Development 1930-1960 vol.2 (1st ed.). Ramsbury: The Crowood Press. ISBN 978-1-86126-939-3."

It appears that T-40 was always used to power a contrarotating prop system except in the case of the XF-84H which had a single prop.

A similar concept was adopted on a bomber used by the German Luftwaffe in WWII.
The bomber, named Heinkel 177 "Greif" was a four engine but had just two propellers since a pair of engines was mechanically coupled in each of the two nacelles.
However coupling mechanically two engines on a single propeller revealed as a source of troubles (e.g. overheating occurred frequently on the rear engines) and the design proved to be unsuccessful. This probably explains why it has not been used any more since then.

Better use the He-119 - this one had two coupled piston engines driving a single propeller. But not turbines.

Both examples are about equally good; the Greif satisfies the question twice (once on each wing)-- except for turbine / piston issue.  Combine both together in same answer.  I'm upvoting because it's interesting even though doesn't fully satisfy the question because not turbine. Also the same configuration was used in the Macchi MC 72 seaplane racer so that could be included as well.  https://en.wikipedia.org/wiki/V24_engine
https://en.wikipedia.org/wiki/Macchi_M.C.72

Oops MC 72's engine was indeed two engines combined into a single shared drive shaft but it drove a pair of counter-rotating props.

Depending on how strict you are about "single rotor" The AH-64 Apache has 2 engines and one main rotor. From Wikipedia "American four-blade, twin-turboshaft attack helicopter"

As does the Black Hawk, both using the T-700.

And the Russian Ka-50/52...though they have a coaxial rotor setup instead of having a tail rotor.  And the Mi-28.....heck....how many military helicopters out there today don't use a dual turbine arrangement..

Kamov Ka-26 (1969, 816 built) is powered by two 325 hp (239 kW) Vedeneyev M-14V-26 radial piston engines mounted in off-board nacelles, connected by a transverse shaft which drives the co-axial rotors.

According to Wikipedia, the reciprocating engines, although more responsive than modern turboshafts, are relatively maintenance intensive. The Ka-26 is underpowered with its two radial engines, especially when used in cropdusting role, where excess payload is common. No other helicopter exists in the world that runs at constant 95% engine power for most of its flight regime. This leaves the pilot with little power reserve for emergencies. Due to frequent overloads, the interconnect shaft which joins the two engines is prone to breakage and requires frequent inspection.
NOTE An edit to the question makes this answer nonresponsive.

Cut little thing. The wheel pants are a nice touch. I wonder how much of whatever's being crop-dusted the pilot ends up breathing? Wouldn't want to fly in it for anything, but, y'know, cute...

@BobJarvis with the main rotor downwash pushing the dispensed chemicals away from the craft, I don't think the pilot is hugely exposed.

The CH-53 uses 2 jet engines to power one rotating wing.

The Fairey Rotodyne probably counts, depending on your definition of "single rotor".
In level flight it was essentially an autogyro, driven by twin turboprops on short wings, deriving a fraction of its lift from these and the rest from the undriven rotor.
To achieve vertical takeoff and landing, the rotors were spun by tip jets, fed from bleed air from the turboprops, augmented by fuel burnt in the tip jets.
Although it seems to have been technically successful both in lifting capability and speed records for rotorcraft, the tip jets made it unacceptably noisy at takeoff and low altitude, precisely where noise was most objectionable.
I wonder if bleed air alone could work nowadays (given higher pressure ratio turbines) to reduce the tip jet noise.

authors: Bob Jarvis - Слава Україні

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What is a Terminal Radar Service Area (TRSA) and what are its operational requirements?
Some fairly busy airports that you would expect to have Class B or C airspace have class D airspace at the airport and normal class E and G airspace around that. In addition, they have an area drawn on the chart with similar dimensions to a class B or class C airspace area but they are labeled as Terminal Radar Service Area (TRSA). What is a Terminal Radar Service Area (TRSA) and what are its operational requirements?

There's some related content of interest in answers to https://aviation.stackexchange.com/q/92849/34686

A good explanation of a Terminal Radar Service Area (TRSA) can be found in the Aeronautical Information Manual 3-5-6.

TRSAs were originally established
  as part of the Terminal Radar Program at
  selected airports. TRSAs were never controlled
  airspace from a regulatory standpoint because the
  establishment of TRSAs was never subject to the
  rulemaking process; consequently, TRSAs are not
  contained in 14 CFR Part 71 nor are there any TRSA
  operating rules in 14 CFR Part 91. Part of the Airport
  Radar Service Area (ARSA) program was to
  eventually replace all TRSAs. However, the ARSA
  requirements became relatively stringent and it was
  subsequently decided that TRSAs would have to
  meet ARSA criteria before they would be converted.
  TRSAs do not fit into any of the U.S. airspace classes;
  therefore, they will continue to be non−Part 71
  airspace areas where participating pilots can receive
  additional radar services which have been redefined
  as TRSA Service.

These airports can't be redrawn as class B or class C airspace so they became TRSAs. The airspace at the airport is class D and the airspace in the TRSA is usually class E. The operational requirements are no different than any other class E or class D airspace, but aircraft are encouraged to avail themselves and participate in the TRSA when inside its bounds.
It should also be noted that many TRSAs have their own approach control.

Note also that many (but not all) TRSAs have their own approach control like a Class C airport.

@NathanG many Class Ds have their own approach control as well, particularly in the Western states, e.g. ASE, BIS, CPR, FSD, FSI, GTF, MIB, ROW, RCA among others. Military airfields are particularly likely to be Class D but have their "own" approach control

A TRSA is an area where pilots have the option of receiving Radar service in a location which has not qualified as a Class C or Class B. As mentioned in @ryan1618's answer, a TRSA  does not change the classification or regulatory nature of the airspace.
I had the creation and operation of TRSAs described to me at an Oshkosh forum by an airline captain.  He explained it something like this:

A TRSA comes about when a radar system is upgraded, and the older unit is appropriated by a congressman for his home district.  He gets it installed at his local Class D airport to the delight of the airport and tower managers.  > When calling the designated approach control, if a pilot is not interested in having the controller tell him what to do, he need only say "negative radar service."  If inbound to land at the delta airport, they may call the tower and hear a voice remarkably like that of "approach control"

If I were a Class D tower operator.  I would think it was cool to have radar coverage at my airport, and it might make me feel like a big-shot to act like I was a TRACON facility.  Indeed, it may even be a safety enhancement and provide a training opportunity to the tower controller.
In my experience, there are some radar equipped Class D airspaces which operate like this without having a TRSA charted.  A clue lies in the existence of an approach frequency in the chart supplement.
See:

KFSD Sioux Falls, SD
KFLO Florence, SC

Interesting stuff. Thanks.


text:
How is %MAC affected by flap/slat position on swept wing, transport category aircraft?
If you watch the slats and flaps deploy on a swept wing, transport category aircraft, and you mentally draw a chord line, you'll notice that it's changing through the slats/flaps deployment schedule. Center of gravity (CG) on these aircraft is usually defined in percentage of mean aerodynamic chord (%MAC). If the chord is changing, is the %MAC referencing a clean configuration? Is the aircraft designed to tolerate the %MAC that exists in other flap configurations? Or is the %MAC constant throughout slat/flap deployment?

In what context is CG measured from %MAC??  I typically see it measured from a datum point at the nose of the A/C.

This question gives you a bit of background on %MAC. In larger aircraft we generally use %MAC as an expression of the CG rather than from a datum point.

I can add more detail for why I ask. I fly a CRJ-900. If you look a the wing configured to flaps 8 you have slats at 20 degress and flaps at 8. The wing chord is obviously different and it's noticable in the airplane when you deploy it. The chord and therefore the %MAC must be affected, and I am wondering how manufacturers approach this.

Ok. Shows my lack of experience in Transport-size a/c. All my experience is just in helicopters and light GA.

The great thing about this site is that there are people with different education and experience that can share insights.

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text:
How should a flight review be logged?
14 CFR 61.56 (c) requires pilots to perform a Flight Review every 24 calendar months unless another applicable action has been taken in order to be eligible to act as pilot in command of an airplane.
The requirements of a flight review state that a minimum of 1 hour of flight instruction and 1 hour of ground instruction are required to complete that review.
My question concerns the logging of that activity. In the past my instructors have given me an signed logbook entry for the flight time, a description of the training received and materials reviewed a logbook endorsement stating that the flight review was performed. Most logbooks have a section for ground instruction, but I've never logged specific ground time there during a flight review. For a flight review to be valid does the ground instruction actually need to be logged separately from the flight instruction? Or is the ground instruction implied by the endorsement?

Short answer: log it however you like as long as you log it.
According to the FAA:

When the applicant has successfully  completed the review, the CFI
  should endorse the pilot’s logbook to certify that the pilot has
  satisfactorily accomplished the flight review.  The CFI should make
  the endorsement for a  satisfactory review in accordance with AC 61-
  65. The flight and ground time must also be  logged in the pilot’s logbook in  accordance with § 61.51(a)(1)

61.51(a)(1) simply says that "Training and aeronautical experience used to meet the requirements for a certificate, rating, or flight review of this part" must be logged.
So you do have to log both ground and flight time, but exactly how you do that is really up to you. The FAA doesn't require any specific logging format as long as the information is correct and CFIs, DPEs and others can see what they need to see.
Personally, I would note the ground time beside the CFI's endorsement and then log the flight time as I would for any other training flight. I've never met anyone who logged their 'ground instruction' time precisely (simulator time is a different story), but if you want to then you can do it any way that works for you. The FAA just wants you to log something that shows you did receive the necessary ground training.

Just use the ground s hook column, and add it to the notes, like you do with student pilots.

rbp, please consider expanding your answer, it got flagged as low quality due to its length.

authors: Мария Гетрик


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What are the details of opening a cockpit door from the outside?
Can one pilot lock the other out of the cockpit?
It is mentioned in the answer to the question I linked that a person inside the cockpit can stop a person outside from opening the door by "canceling the unlock process". This leads to several further questions:

How exactly does the person on the inside "cancel the unlock process" (Press a button, flip a switch, enter "counter code", etc...)
It seems there is a delay before the door opens. Is this correct? What's the duration of the delay?
What are the "authority levels" of codes? Which levels exist, what do they mean?
What is the confidentiality policy regarding those codes in theory and practice? (Who gets what authority level code officially? If stewards/stewardesses are not allowed to have the codes do pilots usually pass them on anyways?)

These sorts of questions would be difficult to answer without potentially exposing information which would compromise the very security measures you're asking about. (Not that security through obscurity / "It's safe because nobody ever talks about these things" is a good policy by any means, but the folks who have this information may be legally/contractually prohibited from sharing some of it.)

Also, the whole idea of the locking cockpit door was to keep intruders (terrorists) from being able to force their way in. Either we keep everyone out, trusting those few needing in, or we don't. We cannot have it both ways. Do we trust the cabin crew with the codes? Mr. Hijacker points a gun at the head of a child and now what?

@CGCampbell: How would that possibly work? If the cabin crew gives Mr. Hijacker the code then everyone is going to die, including the child...

@Mehrdad historically speaking, most hijackers aren't set on suicide. If I were a cabin crew member, I'd much rather not know the code, than have to weigh up one definite death now versus X possible deaths later.

I'm going to explain this for the Airbus (also known as the Cockpit Door Locking System or CDLS):

How exactly does the person on the inside "cancel the unlock
      process" (Press a button, flip a switch, enter "counter code",
      etc...)

If the code is entered for an emergency external opening, the pilot has 30 seconds after the request period to toggle the door button to lock to deny the request or the door will unlock. It pops back the the 'Norm' position automatically.

It seems there is a delay before the door opens. Is this
      correct? What's the duration of the delay?

30 seconds, unless the lock button is pushed, in which case the request is inhibited for 5 minutes.

What are the "authority levels" of codes? Which levels exist, what do
  they mean? What is the confidentiality policy regarding those codes in
  theory and practice? (Who gets what authority level code officially?
  If
      stewards/stewardesses are not allowed to have the codes do pilots
      usually pass them on anyways?)

There are indeed two codes, but I believe the normal access one is optional. The (optional) normal code just chimes the cockpit for an opening request, while a secret code to the best of my knowledge is shared among the crew to request emergency access. "There are two different access request types : “Routine” and “Emergency” access request." These are however internal airline policies and differ from airline to airline, and I've read in some cases that they might even have the same emergency code across the fleet. In any case, the pilot(s) can still decline access.

Here is a video about cockpit door as well.
Source, Page 37 onwards. While the source suggests that the time can be changed, most sources I've read refer to 5 minutes and 30 seconds as the standard times.

-1 for the gif. My eyes!!! (not really, but AARRGGHH!)


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How do the chevrons on engine nacelles reduce noise?
The B787 has these chevron-shaped teeth on the exhaust port of its engines to reduce noise.

And according to the answer to this question they will be incorporated into the new B737's
Can anyone explain how these work to reduce noise?

Ah, thanks. It's a dupe. Can the [tag:noise] tag be added to the original. I didn't find it.


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Why do we call it an apron?
Where did the term apron come from?
Follow up question as suggested by @Pyritie.

Because it's worn by the airport to protect the terminal from grease splatters?

In US apron may refer to the area where the drive of a house meets the road. The "apron" may comes from that meaning, but i dont sure which exist first.

Coming soon to a Hot Network Question list near you: "Why do we call it an airplane?"

It is not an airplane and never has been The word is "aircraft".

@RedSonja: Airplane is correct. Aircraft includes powered fixed-wing (airplane / aeroplane), non-powered fixed-wing (glider), and rotorcraft (helicopter), aerostat, and other things that have wings, boyancy or rotors. See this example of classification for aircraft.

Nah, I used to build aircraft and it was the first thing we learned. :-) Calling it an airplane cost a round of beer.

Airport apron refers to the area of an airport where aircraft are parked, unloaded or loaded, refueled, or boarded and looks like this:

Image from airplane-pictures.net
This probably originated from the word defining theatrical apron:

1903 A. B. WALKLEY Dramatic Criticism 108 The ‘apron’ is the technical name for the stage-area in front of the curtain. In the Elizabethan theatre it jutted right out among the public, who surrounded it on three sides. This ‘apron’ slowly shrank..till at last in our day it has altogether disappeared.

As the airport apron looks similar to this, it probably go its name. The following image shows an apron stage (The Visy Theatre in Brisbane Powerhouse)

Image from gtaust.com
As for why its like this, the following explanation seems the most plausible:

Waay back in the day, airports were simply 1/2 mile or 3/4 mile squares of flat grass-covered former farm field with a small terminal building in one corner. Planes landed & took off on the grass, and parked in front of the building to load & unload.
Given the reality of rain & snow, the area in front of the building would get muddier faster thatn the rest of the field. Between plane traffic, fuel truck traffic, cargo wagon traffic & passengers (& horses), churning the surface to mud would be pretty much a sure thing.
A pretty obvious innovation would be to pave a more-or-less fan-shaped area in front of the building where the airplanes could park.
Which when viewed from above, would look like an apron

As for tarmac, it is simply a synonym for Asphalt, which was used in apron's construction.
Note: The historical term apron bears similarities to modern ramp, as in 'walk the ramp'

But where does the theater apron stage come from? Apparently from another French word napperon (little napkin).

Apron may be the result of a wrong translation from French vocabulary (France had an active role in aviation history).

The circulation area of a bridge is named tablier (roadway) in French. Actually tablier is used to designate many flat engineered surfaces (it comes from Latin tabula, table).

Tablier is also the apron, the overall used for housework.

I suspect the intent of the translation was to use the flat area meaning, but the other sense was used and apron was wrongly retained for the equivalent of a flat area.
In French the airfield apron is aire de stationnement (parking area) but tarmac is also common in medias.

That sounds plausible. Do you have a citation for the suggestion that it was a mis-translation or is it just a personal theory?

@DavidRicherby: Personal theory.

And tarmac, as neat as it sounds on tv is not a material used for ramps - or aprons.

@TomMcW This looks a lot like tarmac (asphalt) to me. Concrete's probably more common but asphalt does seem to be used, especially in cooler climates where it's not going to soften in the heat.

@DavidRicherby asphalt and tarmac are two different things, although in British English they often use the terms interchangeably. Actual tarmac is an older method of construction using tar. Probably wouldn't hold up under big aircraft. But I'm just being pedantic. Feel free to ignore me

@TomMcW distinguishing between Tarmac (aggregate and tar) and asphalt concrete (aggregate and bitumen, which a lot of people also call tar) seems needlessly pedantic. Anyone using the word "tarmac" in the last 50 years is almost certainly using it an a synonym of "asphalt concrete".

@DavidRicherby Hair splitting is one of my specialties!


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How does a pilot make sure they have received all applicable NOTAM's?
I've always wondered how a private pilot makes sure they have read all the NOTAM's that might affect their flight. There must be thousands of NOTAM's out there. This question talks about the many types. Some will be just for the day, some applicable for months. Some affect large areas, some just one taxiway. If doing a cross-country flight, especially to an unfamiliar airport, how does one know how to wade through all of them and be sure they've covered all of those that might affect their flight?

You can use the FAA NOTAM search feature on their website. https://notams.aim.faa.gov/notamSearch/

More recently, tools like Skydemon do a great job of finding the relevant NOTAMs along your planned route.

A quick lookup tool is PilotWeb.
The pilots may receive NOTAMS during flight briefing.  The best way to ensure that you are getting proper NOTAMS is to request all NOTAMS for the route of flight you have filed when you speak with the flight briefer.  The briefers are trained professionals, and they know what NOTAM information to provide to pilots, but it must be requested!
The FAA says this in regards to a standard briefing:

Notices to Airmen (NOTAMs)
(a) Available NOTAM (D) information pertinent to the proposed flight,
  including special use airspace (SUA) NOTAMs for restricted areas,
  aerial refueling, and night vision goggles (NVG).
NOTE- Other SUA NOTAMs (D), such as military operations area (MOA),
  military training route (MTR), and warning area NOTAMs, are considered
  “upon request” briefing items as indicated in paragraph 7-1-4b10(a).
(b) Prohibited Areas P-40, P-49, P-56, and the special flight rules
  area (SFRA) for Washington, DC.
(c) FSS briefers do not provide FDC NOTAM information for special
  instrument approach procedures unless specifically asked. Pilots
  authorized by the FAA to use special instrument approach procedures
  must specifically request FDC NOTAM information for these procedures.
NOTE- NOTAM information may be combined with current conditions when
  the briefer believes it is logical to do so.
NOTE- NOTAM (D) information and FDC NOTAMs which have been published
  in the Notices to Airmen Publication are not included in pilot
  briefings unless a review of this publication is specifically
  requested by the pilot. For complete flight information you are urged
  to review the printed NOTAMs in the Notices to Airmen Publication and
  the A/FD in addition to obtaining a briefing.

If someone is performing a self-brief via 1800wxbrief.com, which is the best internet briefing site in my personal opinion (because it will tell you if you make a mistake rather than purge your flight plan without telling you), they can see NOTAMS invoked by relevance to their proposed flight path.
So to be sure you're getting everything, call 1-800-WXBRIEF via phone and file that way and request NOTAMS from the briefer.
One last thing that needs to be reiterated here:

"This
  question
  talks about the many types [of NOTAMS]."

There are various classifications of NOTAMS, but for all intents and purposes in the civilian world, we have two types of NOTAMS; Distant-(D) and FDC.  That's it.
Civilians haven't had local NOTAMS in the US since 2008 when an advisory circular announced that they were being packaged into NOTAM-(D)s

The briefers you refer to in the first paragraph, is that who you speak to when you call the FSS?

@TomMcW I suppose they are the same organization that operates the FSS.  They are "Lockheed-Martin Flight Services" under a government contract to handle flight services.  1800wxbrief.com is their company's flight plan filing/briefing site, and 1-800-WX-BRIEF is their phone number for the service.  DUAT and DUATS are official, but they aren't as user friendly.  Wingx Pro 7's(a pilot app) flight plan filing system utilizes a 1800WXbrief.com account as opposed to DUATS.

Just an update to the comments for future readers: the FAA has terminated the DUATS program effective May 16th, 2018.

Theoretically, they could go by NOTAM category and select every NOTAM that falls along their planned route of flight (and planned alternates, if applicable), along with a few NOTAMs that warrant a general check regardless e.g. Presidential TFRs anywhere near their route of flight (and I use the term "near" very loosely; I'd check for Presidential TFRs anywhere in the state, and even in the adjacent state if close to the border).
More practically, it depends from pilot to pilot, but if you file a flight plan and ask for a briefing (which you really should for all but the most local of flights), you can ask the briefer to check for NOTAMs in effect along your route of flight to leverage their software to filter out unnecessary info.
More recently, we've also got a plethora of digital flight planning tools that can access the FAA NOTAM databases and pull down any and all relevant NOTAMs according to the filters you set.
At a minimum, you'd want to make sure you might want to check NOTAMs at your airports of departure and destination, along with your alternates and any TFRs. Checking NOTAMs for any navigational equipment you're planning to use (e.g. VOR/DME stations under maintenance, RAIM outages, etc.) is also a good idea.
Ultimately, however, it's up to the PIC to determine which NOTAMs are relevant and should be reviewed and which ones are not. All part of the position's prerogatives.


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When approaching stall, why does the center of pressure move back?
In an answer to a previous question, Peter Kämpf stated:

Before the lift coefficient of a stalling airplane peaks, the flow over part of the wing will start to separate. Ideally it will do so near the trailing edge of the wing root, and the separation will slowly progress forward and outward as angle of attack increases. This separation will shift the local center of pressure back, such that the aircraft will experience an increasing nose-down moment as it approaches stall.

If separation starts at the back and progresses forward it seems to me that center of pressure ought to move forward. Why does it move back?

@mins so this applies only to swept wings?

See page 138, then 156 and following. Actually just before the stall there is a CP move forwards, then a rapid move aft due to the separation at the leading edge (didn't know that) but the latter only on thin wings.

@mins increase tip AoA (twist) or add fences, so that the tip stalls last and the CP moves forwards Do you have this backwards?

In an answer to a previous question, Peter Kämpf stated: -- I wonder how many of our questions start out with that statement... I know I've asked more than one of those.

@FreeMan I was going to say "The great Peter Kämpf." Although it looks like we may have another budding PK in spacegirl1923. Did you read her profile?

@TomMcW I did, I did!

@Peter Kämpf I quoted you here from another question and am trying to understand the concept. But the answers I'm getting seem to be controdicting what you said. Most are saying that the CP moves forward up until the stall then moves backward after stall. If I understand you correctly in the paragraph I quoted, you say that CP moves aftward before you reach critical AoA. (In fact, I left the word off the part I quoted, but you actually said it moves aft long before you reached CLmax. Did I understand you wrong? Does your explanation only apply to swept wings?

It is the overall centre of pressure that includes the stabilizer that moves back.
The reason is that as the flow starts to separate, the lift it generates no longer increases linearly with angle of attack. However, as long as it is properly designed, the elevator is not stalling yet and its lift does still increase linearly with AoA, leading to higher increase of lift on the aft surface and overall down pitch moment.
The centre of pressure of the wing itself should indeed first move forward a little as the trailing edge loses lift. Not too much, because the centre of pressure on the upside is always quite far forward to begin with. And then past $C_{L_{max}}$ it moves aft since the centre of pressure on the underside is about midchord.

the elevator is not stalling yet and its lift does still increase linearly with AoA, leading to higher increase of lift on the aft surface and overall down pitch moment. But the HS is producing a downforce, especially at a high AoA. Would a higher negative lift not shift overall CP forward causing a pitch up?

@TomMcW, lift is an upforce. If it is actually producing a downforce (usually is), increasing lift means it now produces less downforce, not more.

The center of pressure is generally not the best way to characterize the aerodynamic forces on an aircraft since it varies with AoA.  That is why the neutral point or aerodynamic center is used instead.
Having said that the center of pressure is the location of average pressure force acting on the wing or aircraft.  Mathematically it is found by taking the sum of integral of the magnitudes of pressure and shear stress acting on the surface and multiplying it by a distance (for an airfoil from the LE) on the upper and lower surfaces resulting in a moment at the LE, or reference point.  This is then divided by the sum of the integrals of the pressure and shear stresses acting on the upper and lower surfaces.  My explanation does not do it justice see http://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf
It can be somewhat complicated to calculate for an entire aircraft but in general for an airfoil it does move forward up until the point of stall or critical point after which it then moves aft.  See https://www.reddit.com/r/aviation/comments/2kibfo/why_does_the_centre_of_pressure_move_rearward/ for a good simple explanation of this.
The movement of center of pressure is highly dependent on the configuration of the aircraft (airfoils are much simpler) and unless you are certain of the specifics of the configuration generalizing the center of pressure movement is inaccurate...

The movement of overall centre of pressure is dependent on configuration, but as long as you assume the horizontal stabilizer does not stall yet, the general tendency can be derived.

General tendencies for a traditional configurations yes, but once you start adding in wing sweep, twist, canards, and less traditional configurations, generalizations about movement of center of pressure are not a good idea.

The conditions for down pitch moment when approaching stall are closely aligned to conditions for stability. So while it is certainly possible to come up with design that will behave differently, stable designs will generally pitch down when approaching to stall.


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How do trainer aircraft differ from non-trainer versions?
In flying schools, there are aircraft designated for training student pilots. Do these aircraft have special equipments or controls on-board which are used by instructors?
Like maybe extra panels or something?

One of the requirements is that the aircraft is controllable from both seats. One of our colleagues could not use his own plane for training because the brakes were only controllable from the left seat, so the instructor would not have had any.

possible duplicate of What are the properties of a good general aviation training aircraft?

The short answer is that there's no real difference between a "trainer" aircraft and an "owner" aircraft when they roll off the manufacturer's assembly line.
Below are two mid-1970s Piper Cherokee Cruisers - take my word for it that they both have virtually identical instruments, avionics, and interior finishing. One of these airplanes is the "trainer" I did much of my early training in, and had a little over 11,000 hours of flight time as of when the school sold it. The other is (probably) an owner-flown airplane with about 2,000 hours of flight time.

Can you tell which is which? The answer is in the spoiler box at the end of this post.

"Trainer" aircraft generally have flight and operational characteristics that makes them "suitable for use in training" -- though what that means is a subject of some debate.
In modern training aircraft you'll almost always find the following:

Dual controls
Much like a Driver's Education car has an extra brake, aircraft used for training will have dual controls so your flight instructor can correct any mistakes the student makes before they become dangerous.  (Dual controls are in fact required for flight training, at least under FAA regulations, though interestingly dual brakes are not required.)
2-4 person seating capacity
Hand in hand with dual controls goes dual seating - You need a place for both a student and an instructor.
You don't generally find many trainers that will seat more than four people, as those larger aircraft cost more to purchase and operate.
Some kind of intercom
Students and instructors generally wear headsets, and an intercom system makes it easier for them to communicate.
One or more Radio systems
Part of flight training includes learning how to communicate with Air Traffic Control and other pilots, and how to use radio navigation systems. This requires at least a communication radio, so there's usually one either installed or available (a handheld unit), and in most modern trainers you'll find one or more navigation radios of various types.

In addition trainer aircraft tend to be inexpensive to maintain (mechanically simple), operate (relatively fuel-efficient), and tend toward the slower end of the speed spectrum (because it's easier for a student to keep up with a slower aircraft).

It's important to note that not all "trainers" fit this mold: The T-6 Texan is, by designation, a Trainer aircraft, but its goal was to train Air Force pilots, many of whom would go on to air combat roles. The things a T-6 is optimized to teach are very different from what a Piper Cherokee or Cessna 172 are optimized to teach.

Not surprisingly many of those same characteristics are also desirable in a plane you might buy to fly yourself: Dual controls in case you take a friend or another pilot along (or in case you want to take instruction in your own aircraft), an intercom to let you talk to your passengers, and radios so you can navigate and communicate in the air traffic system.
Because of this many pilots who elect to buy an airplane purchase what we would consider "trainer-class" aircraft, frequently models similar to the ones they trained in (and in the used market, frequently aircraft that were used for flight training at some point).
When you leave the "trainer" range you tend to find faster aircraft, more complicated systems, more attractive finishing elements (plush cabin interiors, fancy paint schemes), and more seating capacity.
You also find aircraft that begin to specialize more: Where trainers tend to be well-rounded the "non-trainer" planes tend to excel in specific areas, serving specific niches in the market (the Extra 300 is a high-performance aerobatic aircraft favored by many airshow performers, Mooney aircraft feature extensive speed optimizations and are favored by many pilots who use their planes for long-distance travel, the Piper Mirage is a "luxury" cruiser/transportation aircraft suitable for flying corporate executives to and from smaller airports, etc.).

 N2269Q is the trainer, now sporting a shiny new paint job.
 N32711 is the owner-flown airplane, with its factory paint. (Trade-a-Plane ad)

The control situation in particular is very different than with cars - while cars are designed so only one person can drive them, it's very common for aircraft to have two sets of controls if they have two side-by-side seats up front (for whatever reason; one possibility is that unlike cars, you can't just pull a plane over and switch who's in the pilot's seat if you want to switch who's flying).

Left one is trainer since it is in use. Right one is the owner plane. It is well kept in a nice clean hangar!!

Do I get any bonus for guessing correct?

Would it be worth mentioning aircraft used for higher-level training, such as twin ratings, instrument ratings etc?

@DJClayworth I thought about it but decided to confine the answer mainly to primary trainers. There's certainly more that goes into a good multi-engine trainer, but a (single-engine) IFR trainer and a basic owner-flown (single-engine) IFR aircraft are often pretty similar (Two radios, one of which might be a GPS, a transponder, heated pitot tube, etc..)

The T-6 example is complicated. The AAF in WWII used three different aircraft, in this order, to train pilots: Stearman PT-17 (primary trainer), the Consolidated Vultee BT-13 (basic trainer), and then the AT-6 Texan (advanced trainer). They train Army Air Force pilots, not Air Force pilots, since the Air Force didn't yet exist. The Navy used the SNJ-4/-5-/-6 variant of the AT-6 to train its Naval Aviators.

Mooneys are no longer being manufactured, and the current Cessna TTx is faster than the fastest Mooney.

@rbp Mooneys are in fact being manufactured. They resumed production last February (they're even bringing out new designs) - I'll concede the speed point though, I forgot about the TTx :)

@rbp - that was the T-6 Texan. The Air Force (and Navy) currently fly the T-6 Texan II, which is a essentially a Pilatus PC-9 built by Beechcraft. It is a basic/primary trainer, with follow-on in the advanced trainers (T-1, T-38, UH-1)

To give a general answer, it is important to define what makes an aircraft suitable as a trainer:

It must load at least two people. But also rarely more than two. Therefore, most trainers are two-seaters or half-empty four seaters. They come with dual controls and - in case of tandem seating - two sets of instruments, and none to little modification is needed.
It must fly slowly and be naturally stable. Slow aircraft leave you more time to do the flying, traffic monitoring, radio communication and what else is asked of a proper pilot. Natural stability makes flying much easier, and it helps to avoid the cost and complexity of a computerized flight control system which could compensate for the instability. Propeller aircraft of a conventional layout fit well here, and only advanced training is done on jets.
It must be cheap to operate. The physics of flying does not change with size, but the cost per flight hour certainly does. Again, GA propeller aircraft are a good fit.

Now look at what those pilots will fly later in their lives:

Recreational flying: Mostly the same GA aircraft, or more advanced, but still broadly similar types.
Professional civil pilots: Big turboprops or even bigger jets. If they have to earn a profit, aircraft need to be big and fast. Both characteristics disqualify them for training apart from getting a type certificate. Since they also have dual controls and are designed to be flown from both seats, they can be used for training without any modification at all.
Military pilots: They fly either similar types like their civilian colleagues, or highly specialized machines with relaxed static stability, extreme power-to-weight ratios and complex electronic equipment. Now it depends what your definition of a "normal" airplane is - but this category of aircraft is very different from trainers. In this category there are indeed special two-seater versions especially for pilot training. In most cases, a second cockpit was added at the expense of internal tank volume, so the equipment is fully functional and identical to the combat version. See the examples below:

Single-seat "regular" Eurofighter (above) and the two-seater training version (below)

Single-seat "regular" MiG-21 (right) and the two-seater training version (left)

Some military planes have no 2-seat versions - they train in dedicated trainer aircraft, get rated in those, and then spend time in the simulators before flying their final aircraft (usually with a qualified instructor flying chase in another identical airplane)

Additionally, some of the 2-seat fighters aren't trainers - they have the second seat for a Weapons System Officer, though the plane can be flown from there


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Why do some aircraft have 4 nosewheel doors?
The A380 has 4 nose-wheel doors. Look:

Source: Airliners.net
Why does it have 4 nose-wheel doors?

Possibly because one giant one would be a lot of load on a single door? Think of physics, the further away from a pivot any force applied multiplies by the distance. If the door were one giant door (considering it has to have the same opening size), the door hinge would have to be significantly bigger (more complicated).

Actually there are 4 doors! Is that what you mean? Could be due to the curvature of the nose, and @RonBeyer comment is valid too. This is cool for souvenirs. Not a record though.

To add on to @mins comment, many other aircraft for all different purposes have 4 doors for the nose gear.

@SMSvonderTann Yes! The 757 has another nosewheel door, for example

Having multiple door sections for the nose gear is a common design. Looking closely at the photo below may make the reason more clear:

When the nose gear retracts, it swings forward. You can see that the wheels will need a larger opening than the narrower strut. So instead of having one door with a strange shape or that is larger than it needs to be, it is split into two sections. This also allows the forward doors to be closed once the gear is extended, which reduces drag.
Splitting the doors down the middle allows for symmetry, and it reduces the amount of surface area each hinge or actuator must support. This results in 4 separate door sections.
Looking at the main landing gear, you can see it uses a similar design. For example, the outboard gears swing inwards on retraction. The doors that cover the main gear bays are larger, and are only open during extension or retraction, and are otherwise closed. The smaller doors at the locations of the struts remain open.

Please provide proper attribution of your photo.


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What is the IATA code for the A320neo family?
I was looking for A319neo, A320neo and A321neo IATA code all over the net but I could't find it, can someone help me with this.
(As of 9 March, 2018, the answer with the highest up-vote count (from April 2016) states "will likely be the same as". A more definitive answer should be available now.)

The IATA three letter code for the Neo will likely be the same as for the regular model.
Unfortunately the IATA code list is copyrighted, and I can't find a public document that confirms it.
The more commonly used ICAO type designator, a four letter code can be found in  ICAO doc 8643. It contains a list of all common aircraft and their codes.
The ICAO type designator for the neo will be the same as for the regular versions. So A319, A320 and A321. Only if the performance of the Neo aircraft significantly differs from the originals, then a dedicated designator will be created.

Note that NEO is, despite its business importance, very small change of the design. A318–A320 don't even get new revision number; they are still A318-100, A319-100 and A320-200 (A320 was revised early on, before they even started to build the other sizes). Only A321 is upgraded to -200, because with winglets it seems to be getting the series standard wings instead of the modified ones of A321-100. And the ICAO and IATA identifiers don't make difference between A320-100 and A320-200, so they are unlikely to make it between A321-100 and A321-200 and even less so between A320-211 and A320-251N.

The IATA code for A320 with wingtip fences is 320. The code for the Sharklet equipped A320s is 32A. I'm sure the Neo will get a new code.

There are now ICAO designators for the A3XX Neo aircraft:

A319 Neo: A19N
A320 Neo: A20N
A321 Neo: A21N

Source

For completeness, do you have a reference for that?

While this is interesting, it does not actually answer the original question.

As mentioned by @CrabLab:

Reference: A document by ICAO.

This doesn't answer the question. The question asks for the IATA aircraft type code. This is the ICAO aircraft type designator. They differ. For example, a A300-600F has a ICAO code of A306, but a IATA code of ABY. ICAO codes are separated by aircraft performance for ATC purposes, IATA codes are for reservation and booking systems.


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Has a general aviation twin-engine turboprop with counter rotating propellors ever been built?
In my experience with general aviation twin-engine turboprops I've never heard of one with counter rotating propellors. I've never seen any data about an engine manufacturer building left-rotating turboprop engines.
I'm also curious if any counter rotating twin-engine turboprop aircraft exist outside general aviation as well.

The only aircraft with turboprop counter rotating propellers that I can think of is the Piaggio P 180 Avanti, whose design is unusual in so many ways.
Check out http://www.mpa.piaggioaerospace.it/, which gives the specifications of a Maritime Patrol Aircraft(MPA) based on the P 180.

Interesting! I didn't know the P.180 has counter rotating props. Thanks for that.

I wonder how they achieve the left hand rotation. Is it reversed by the gearbox or is the whole engine built reversed?

In these cases, only the gearbox is reversed; rebuilding the engine is too complicated and simply not worth it.

I wonder if one gear box is heavier than the other.

I too would be interested to know more about the gearbox differences.

Damn beautiful aircraft.


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Could passengers on an airline flight create an out-of-CG situation by moving around the airplane?
I've often wondered if most passenger airliners have a CG envelope which would allow the CG of the airplane to be pushed outside the limits if enough passengers relocated to the front or back of the airplane.
If they do passengers might be surprised to know that if for some reason a lot of people got up and crammed into the back of the plane it might become uncontrollable.
If this is the case, the mental exercise could help illustrate how important an understanding of weight and balance is. It also highlights the importance of flight attendants as part of the air crew in their need to ultimately exert control over the crowd of passengers if the situation warrants it.

As a bit of background, I once wondered if Flight 93 during the September 11th attacks might have become uncontrollable because the CG shifted when passengers moved around the airplane. Although I once speculated that this could be the case, I am no longer speculating on the subject. I just want to mention that this is the scenario which made me wonder about the answer to the actual question. Since writing the original version of the question I learned about the CVR from Flight 93 and that it was intentionally flown into terrain by the hijackers.

I removed the speculation with a tenuous link to the question since we will never know.

@Simon Actually, we do know and the speculation in the original post was definitely incorrect. Several of the passengers and flight attendants made cell phone calls and described what was going on (and learned about the earlier crashes.) Furthermore, the black boxes were recovered and showed the control inputs and recorded the cockpit conversations.

You guys marked it a duplicate when it's not. I asked if passenger movement can move the CG out of limits, not if it would make the airplane stall.

I also think just deleting a whole section of my question is out of line, Simon.

@RyanBurnette It's a community curated site.  If you disagree, edit it back in.  the community will decide.  However, we do not welcome speculation of any sort.

Alright I added the background back in stressing that I am no longer speculating about the topic. I think it's appropriate for me to say that I once speculated and it is what lead me to ask the question.

questions can be marked as duplicates also if the other question is not verbatim the same, but the answers there contain the answer to the closed one (as in this case)

@Federico fair enough

Respectfully, I voted to reopen the question because in my opinion this question is sufficiently different from that of the question asking if passenger movement could cause a stall. The two questions have, again in my opinion, opposing answers. Looking at just the question titles, the practical answer to this question is yes, The practical answer to the question about causing a stall is no. In both questions, of course, you can think of extreme scenarios that would reverse those answers.

@ryan1618, I think it's a very intriguing question. I'm not an expert, but as I've found answers here unsatisfying, I've given it a shot at the question this is a "duplicate" of, you might want to have a look. Short answer: yes.

I was on a flight in an SWA B737 that was mostly empty and the flight attendants moved us all toward the front. They told us it was for balance reasons.
I don't think this was a factor on flight 93. The CVR recorded the hijackers discussing what to do about the passengers. The hijacker piloting the plane asked 'Is that it? Shall we finish it off?' Another hijacker responded, 'No. Not yet. When they all come, we finish it off.' Then a few seconds later when it sounds as if the passengers are getting through the cockpit door he asks, "Is that it? I mean, shall we put it down?" The other responded, "Yes, put it in it, and pull it down." That's somewhat odd phrasing but those are translations from Arabic. It's clear that it was intentional.

I wasn't aware there was a CVR from flight 93. Those are chilling details.

Also, while I appreciate the info about flight 93, this answer doesn't address the actual question directly.

Yes, i see that your question is about passenger movement in-flight whereas my experience was pre-flight. It does illustrate that the crew was concerned about passenger location in relation to cg.

I've also been on a mostly empty flight, where the pilot told us to not change seat until we were in the air because it would unbalance the aircraft.


text:
What is the equality theory, or equal transit time theory, and is it supported by scientific evidence?
Throughout flight training when learning about the mechanics of lift I read and was taught this story about two molecules that part ways at the wing tip and reach the trailing edge of a wing at the exact same time. Later on I heard this referred to as the "equality theory."
The idea is that a wing makes a molecule travel farther to end up at the trailing edge at the same time rendering a greater velocity and a lower pressure through Bernoulli's principle.
I'd like to find the best articulation of the equality theory as well as information on whether scientific research actually supports it. Whether it's scientifically supported or not, it may at least serve as a useful teaching tool. In any case, if I talk about it I'd like to be able to cite something truly scientific.

Its called the Equal Transit Time Theory, and its wrong.

see also here: https://aviation.stackexchange.com/a/16198/1467
here: http://www.av8n.com/how/htm/airfoils.html
and here: https://aviation.stackexchange.com/q/21664/1467
and: http://aviation.stackexchange.com/q/8281/1467

It's interesting how this kind of stuff propagates. I once found a mid-20th-century textbook on the theory of flight that had a figure of streamlines around a wing with a caption saying this was how wings produced lift. The figure appeared to be an exact copy of one in an earlier, more mathematical treatment. The funny thing is, the apparent source had multiple diagrams of the wing showing different air flows we might conceive of existing around a wing; the one the textbook had copied was the "no lift" figure.

Visual demonstration of fallacy in a wind tunnel (Youtube).


text:
What is the FAA's source for the type designators they use in IACRA?
When using the FAA's IACRA system, an online portal used for creating applications for the issuance of a rating, you often have to choose an aircraft type to meet qualifications or to set which aircraft you'll be using during a practical test. When selecting aircraft in IACRA the type designators are different from what I'm used to. For example, a Cessna 172 K model's type designator in my experience is C172K, but in IACRA it's CE-172K. The Beechcraft Sierra would normally be B24R, but it's something like BE-23-BE24R in IACRA.
Where is the FAA coming up with these type designators they're using in IACRA?

I'm not sure if type identifier is the right word for CE-172K. Please help me improve the nomenclature if possible.

Where do you see these "official" abbreviations used?

I've seen them in IACRA. On type ratings. In other pilot's Airline Apps. Occasionally on flight plans. For example the Piper Cheyenne I is a PA-31-500T in one place then PAY1 in another. Just trying to find definitions for the identifiers and associated nomenclature.

Did you use the word "official"? I looked at IACRA and I saw it there, not sure where it came from though. I'll do a little research and see if I can figure it out though.

It looks like the FAA calls it "model designation". It can be found in the type certificate data sheet (TCDS). There is a relationship between "type rating designation" and "model designation". Some designations can be found in this Excel file.

I found this ICAO list of "type designations". http://www.icao.int/publications/DOC8643/Pages/default.aspx

@mins based on that "172, A, B, C, D, E, F (T-41A), G, H (T-41A), I, K, L, M, N, P, Q, R, S = 3A12"  shouldn't it then be 3A12 and not CE-172?

So far, I have checked the ICAO type designators, the FAA type designators and the other resources seen here and I have yet to find this CE-172 anywhere.  Good question.

@RyanMortensen: That's not clear to me. I understand 3A12 is the TC number, which encompasses different models.

Try this.
This is the FAA's Type Certification Data Base, which lists all type certificates issued by the FAA.
Note:  this only covers certified aircraft and not experimental or homebuilts.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/Frameset?OpenPage

You answered before I rewrote the question, but this is a good resource. Thank you.


text:
Are there ILS approaches where descending by the glide slope outside final might be dangerous?
Are there ILS approaches out there where descending by the glide slope on a segment of the approach that's outside the final approach segment would cause the aircraft to descend below the no lower than altitude for the segment?
Here's why I'm asking. It's not uncommon for pilots to intercept the glide slope before the published glide slope intercept point and then to track it all the way down. If doing this could be dangerous or might not provide the protections ILS approach are designed to provide, it might be a better procedure to descend using step downs to the no lower than altitude for each segment only intercepting the glide slope at the published point for all approaches.

Are you talking about descending on extended final, or on some other leg?

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.

@RonBeyer I'm talking about descending on a segment of the approach, but outside the final approach segment.

As your answer says, it is never safe to descend below the MDA published for the particular segment you are tracking, regardless of you having a glideslope signal. Think about coming into an airport in mountainous terrain. You may hit a peak that the MDA step-downs are published to avoid if you just follow glideslope.

The answer is simple - every single one

If you don't intercept at the marked point, you run the risk of intercepting a false glideslope. Possible duplicate of What is the behavior of false glideslope signals?

I get everything you guys are saying in the comments, it's why I asked the question. I'm trying to find an approach where descending by the glide slope on an approach segment would clearly bust the lowest altitude for the segment and fail to provide obstacle clearance. If it doesn't exist then it's hard to back up this claim that it's only safe to intercept at the marked intercept point. I'm also trying to decide if I should teach instrument students to descend to the lowest altitude on each segment and only intercept at the marked point or to intercept early in some cases.

I would (and do) teach pilots to intercept and follow the glideslope before the precision FAF.  I also teach them they need to be aware of the MEA's for the segments up to the FAF and to level off, if necessary, to keep from descending too early especially in warmer weather.

The vote to close is entirely misguided... this question isn't asking about false glideslopes, which is the topic of the "duplicate" question.  Entirely different questions!!!

@RyanBurnette  You probably won't find one of those (at least in the US), because if intercepting the GS is likely to run you into or near the rocks, the FAA probably won't install as GS there. (Or they might raise the angle.) There may be a case where an airport is in enough of a "bowl" where that could be the case, and I'd expect bold print warnings on the approach chart. That said, some approaches WILL have you clip the stepdown altitude when tracking the GS. LAS 25L did, although I think the last redesign fixed this. Most won't, though.

Are there ILS approaches out there where descending by the glide slope on a segment of the approach that's outside the final approach segment would cause the aircraft to descend below the no lower than altitude for the segment?

Yes.  See InFO 11009.
In part the InFO reads:

On ILS approaches, stepdown fixes are established for obstacle or traffic separation.  For all practical purposes, the glide slope remains stationary regardless of atmospheric temperature and pressure.  Conversely, stepdown fixes are published for a pilot to fly using indicated altitude, which varies with temperature and pressure changes.  Therefore, the proximity of stepdown fixes in reference to the glide slope, changes with the weather.

We know the old adage:

From high (hot) to low (cold) look out below.

What isn't obvious about that adage is the perspective of that statement.  It is from the perspective of the pilot to the ground.  A glide slope's perspective, if you will, is from the ground to the pilot.  Thus what you will experience is this.  In warm weather, the stepdown altitudes will be higher than the glideslope and you will need to level off to remain above the MEA until crossing the next stepdown fix.
You must remain above the MEA at all times till crossing the precision final approach fix.

From the InFO Examples of airports where multiple altitude deviations have occurred include, but are not limited to; LAX, ORD, ATL, SLC

This information seems to suggest that descent via glideslope I might be dangerous to one's career, but not to safety of flight.

@JW That's not the right takeaway. It's not dangerous from a terrain clearance standpoint but it very well could be dangerous from a traffic separation standpoint.

The minimum altitude of route segments is given in indicated altitudes, while the actual glideslope is effectively fixed in space. What this means is that on a sufficiently hot day following the glideslope will cause you to bust altitude restrictions outside the FAF.
It might be tempting to conclude that this is a technical violation but not a safety concern. After all, if the glideslope does not intersect a mountain on a cold day it won't intersect a mountain on a hot day either. But that misses one important factor. From InFO 11009:

On ILS approaches, stepdown fixes are established for obstacle or traffic separation.

If you bust an altitude restriction due to following the glideslope, you may not compromise your separation from terrain, but you may compromise your separation from other aircraft. These aircraft are (correctly) flying higher than they would on a colder day, and you following the glideslope may put you closer to them than legal requirements and safety allow.
For an example, here is ILS OR LOC RWY 6R at KLAX:

The stepdown fix at OTTES has an altitude restriction of 3600. On a standard day, the glideslope altitude at that distance (11.1 nm from the threshold, TCH 53 feet, GPA 3.0°, TDZE 116 feet) gives about 3700 feet MSL. No problem! You can intercept the glideslope before OTTES and follow it all the way in.
What about on a hot day? At 40°C, assuming a standard lapse rate, the glideslope altitude at OTTES is still at a true altitude of 3700 feet. But the indicated altitude is only around 3400 feet. So if you do the same thing, you'll bust the restriction at OTTES by 200 feet.
OTTES is out over the ocean so terrain clearance is not an issue to begin with. It provides traffic separation instead. Suppose another aircraft is on the feeder route from KILIE for ILS OR LOC RWY 7L:

They are flying at an indicated altitude of 2600 feet and cross basically right under OTTES. If you are following the glideslope at OTTES you will only be 800 feet above them- this is a loss of separation.


text:
What is allowed with the new EASA SPL license?
Are you allowed to fly for profit with SPL? If I have understood correctly, SPL is to LAPL(S) as CPL is to PPL, but with no extra requirements over LAPL(S), but I have problems believing this.
Assuming I have understood the rules correctly, I can upgrade my MGPL to LAPL(S)+TMG and therefore to SPL+TMG with no extra effort. After doing the conversion and flying PIC for 75 hours, I would legally be allowed to ask the passengers to pay for my flights. This seems to be too far from the current rulings that I have trouble believing this.
SPL = Sailplane Pilot License
LAPL(S) = Lightweight Aircraft Pilot License (Sailplane)
CPL = Commercial Pilot License
PPL = Private Pilot License
TMG = Touring Motor Glider

I know very little about this, but it looks like you can fly commercially with an SPL if you pass an additional checkride. This is from the UK CAA, and the same wording is quoted on a bunch of European gliding organization sites:

You cannot operate commercially, or receive any kind of payment, until
  you:

Are 18 years of age or over
Have completed 75 hours of flight time or 200 launches, after your licence has been issued
Have passed a proficiency check with an examiner

EASA's Part-FCL guidance defines what the additional checkride includes:

AMC1 FCL.205.S(b) SPL — Privileges and conditions
CONTENTS OF THE PROFICIENCY CHECK FOR THE EXTENSION OF SPL PRIVILEGES TO EXERCISE COMMERCIAL PRIVILEGES ON A SAILPLANE

Note that it calls it an "extension" of SPL privileges, which strongly implies that an SPL by itself doesn't allow commercial privileges. There is one exception, which is that an instructor can be paid to instruct with just an SPL. The CAA site says:

The exception to this is for holders of a SPL with instructor or
  examiner privileges, who can be paid for the following:

Flight instruction
Conducting skill tests and proficiency checks

for the LAPL (S) or SPL and any associated ratings and endorsements.

Nice job resurrecting an old question with a good answer!


text:
How does a helicopter pilot execute an auto-rotation landing?
What are the critical steps a pilot follows to perform an auto-rotation landing?

@CGCampbell I agree, I edited the question to focus on the process of executing the landing, which I think (I may be wrong) is what the OP intended

Gentlemen, I actually I am looking for specific procedures and why these procedures are done in order to accomplish a successful  engine out autorotation landing. .... i.e. immediately lower the collective, establish a rate of descent, etc. thank you

What do you mean by critical?  Do you mean survive, or 100% successful outcome?

Gentleman , by using the word  " critical"  I am trying say,....having any reasonable chance  at significantly lessening the impact and  all aboard surviving,... this is also assuming there is enough altitude  when the engine failure  occurs, and a suitable  place for landing are not concerns.....thank you

I am going to assume that by "critical", you mean survivable.  I will also assume that the engine failure occurs at a height which will kill you.
It's not so much critical steps, it's critical outcomes.  The single most critical outcome is to preserve, or regain, rotor RPM.  What steps are needed to do this will depend on what the aircraft is doing when the engine quits and, to a lesser extent, the type of the helicopter.  I will generalise for an "average" helicopter in straight and level flight.
The immediate action that is drilled into everyone from the very earliest hours as a student is to lower the collective.  Doing this has three effects.  It removes the positive pitch from the blades which removes most of the drag, it aligns the rotor thrust vector with the axis of rotor rotation therefore not using rotor energy for anything except lift and it causes a clutch to disengage the rotor from the engine allowing it to freewheel.  From this point onwards, you are heading back towards earth.
The second immediate action at the same time as lowering the collective (assuming forward flight) is to pull back on the cyclic to flare.  This loads the disc which causes it to "cone" upwards which reduces the disc diameter.  Therefore, the centre of gravity of the disc moves inwards and due to conservation of angular momentum, the rotor RPM increases.  For quite complex reasons, the nose of the helicopter will also pitch down when the collective is lowered so pulling back counters that tendency.
The third immediate action is to push in the pedal on the opposite side to the rotation of the blades. If the blades rotate counter clockwise (to the left as the pilot sees it) you push in right pedal to reduce the thrust which is produced by the tail rotor which is no longer countering the yaw caused by the drag from the powered rotor.  This is less critical than the first two and although it can be dangerous and get you into an uncomfortable attitude, it is usually possible to recover from not pushing the pedal in immediately.  If you've got rotor RPM, then you can sort it out.
Now you have successfully entered autorotation.  From here, fly more or less normally to the next critical point which is about 50' off the ground.
What you've done is to ensure that the rotor has flying RPM and that you are managing energy by trading potential energy (height) for kinetic energy (rotor RPM).  The conversion is done by the airflow which now comes from under the disc and "drives" the rotor to maintain RPM.  The pitch is neutral, or maybe even negative but, the relative airflow is now upwards through the disc and therefore, the blades have a positive angle of attack and generate some lift.  This stops the helicopter from falling.  There is some drag generated as a consequence of generating that lift, but it is easily overcome by the power which is now driving the rotor from that upward airflow.
As long as you are descending, that conversion will happen and your RPM will be maintained.  The controls are rigged such that with the collective fully down, RPM will remain in the normal range.  Sometimes you have to tweak it a little with small amounts of collective, flares and turns but in general, you just fly towards your landing spot.  The allowed RPM range is greater in autorotation.  For example (and from memory), the R22 has a range of 97-103% in normal flight and 90-110% in autorotation.
You are now descending with a high rate of descent and usually, significant forward speed.  You need to reduce both of those to get to a safe arrival. To do this, there are three more critical steps.
Starting at about 50 feet (depending on lots of factors but let's stick with the average helicopter which entered autorotation from straight and level with significant altitude), you flare the aircraft by pulling back on the cyclic.  This will immediately begin to slow the aircraft.  It will also begin to increase RPM (you are now converting speed into rotor kinetic energy).
At the same time, you increase collective to reduce the rate of descent by increasing the lift generated.  This will rapidly increase the drag, but now the energy needed to maintain the RPM is coming from the flare which is converting speed into RPM.  You also need to put in pedal to stop the aircraft from yawing as the drag increases on the rotor.
Providing you get the entry right and your flare reduces the speed and rate of descent to something survivable, then you will walk away.  You might wreck the helicopter and break some bones, but arrive at 10 feet with only 20 knots and 150 feet per minute rate and you'll get away with it.
If you are well trained and in practice, then you will land safely and smoothly with no damage to  machine or people.
In summary, critical steps:

Entry.  Lever down, cyclic back, pedal in.
Arrival.  Cyclic back, lever up, pedal in.

If we're trying to explain things to the layperson, phrases like "dump the collective" don't mean too much.

@Jamiec Thanks.  I've made some edits.  I'd welcome any more input on "jargon".

On the contrary, I thought the rest of this answer was concise and understandable.

@ Simon....this is exactly the kind of knowledgeable answer I was hoping for...thank you sir....I love this site.....

@garyv440 You're most welcome.

This is not intended to be anything like a full answer, but rather a lay-engineers comment to tack onto Simon’s very nice explanation.
When in descent mode with cyclic down the blades are in negative pitch to usual and airflow through them adds energy to them (up to some controlled limit) rather than transferring energy from them. The rotor becomes an energy store — a “winged flywheel.” Having the rotor at maximum allowed RPM in this mode maximises stored energy. Once you reach maximum allowed RPM the blades can be operated to maximise falling drag, subject to maintaining rotational speed — the rotor is akin to a large flat-plate. the machine will now fall at terminal velocity* for the maximum drag combination.
When you “flare” and raise the collective, blade pitch again becomes positive and you again have a powered helicopter. BUT it is powered by the inertial energy stored in the mass of the rotating blade assembly and you are going to use that up extremely fast — you have a few seconds of flying time, with rotor speed dropping as energy is taken from it. The flare procedure is designed to use the rotational stored energy in a manner which optimises the transition from pre flare velocities to post flare velocities.

* Auto-rotation terminal velocity:
I have not researched this so there may be reasons that it is wrong, but based on many other scenarios for bluff falling objects it seems likely that fall rate will be close to what is predicted by the classic drag equation so Rotor falling drag or machine mass is defined by
$$\frac{1}{2} \rho C_d A V^2$$
where

$\rho =$ air density ($1.2\ \mathrm{kg/m^3}$ near sea level)
$A =$ area ($\mathrm{m}^2$)
$V =$ velocity  ($\mathrm{m/s}$)
$C_d =$ drag coefficient relative to flat plate, say 0.8 in this case

Therefore $\mathrm{mass} \cdot g = 0.6 \cdot 0.8 \cdot A \cdot V^2$
and terminal velocity
$$V_{\mathrm{terminal}} = \sqrt{\frac{\mathrm{mass} \cdot 9.8}{A/2}}$$
Terminal auto-rotate velocity in $\mathrm{m/s}$ is then
$$\sqrt{\frac{20 \cdot \mathrm{mass_{gross}}}{A_{\mathrm{rotor\ disk}}}}$$
While this is a guesstimate based on a range of assumptions, the general principle gives satisfyingly good albeit approximate results for objects as diverse as field, mice, bowling balls, skydivers and cargo parachutes.
(It only works for raindrops when you realise that they generally fall as a flattened disk like shape when at terminal velocity.)
Example:
Robinson R22 Beta II, 620 kg gross weight, 151 inch rotor radius.
Use 600 kg and 46 square metre rotor-disk area:
$$V_t = \sqrt{\frac{20 \cdot 600}{46}} = 16\ \mathrm{m/s} = 58\ \mathrm{km/h}$$

Looking further …
I can see I shouldn’t have started this. Fascinating. Time eating.
The above formula turns out to give a somewhat high auto-rotate velocity, which is good.  Probably 50%+ higher than actual. Possibly due to lift from disk in forward gliding flight.
1,000 feet per minutes $\approx 5\ \mathrm{m/s}$.
Various pages mention auto-rotate descent figures of 1,300 to 1,800 fpm.
Auto-rotation related calculator and MUCH MUCH more - superb.  Includes comments -

A rotor in vertical autorotation has the same resistance as a parachute of the same diameter. This rate of descent is also approximately twice the hover-induced velocity.
2500 ft/min is a reasonable upper limit for larger helicopters, i.e., 13 m/s
The $t/k$ ratio, which is the time in seconds that a rotor can lift the chopper when the engine stops. It is the ratio of $J\cdot\Omega^2$ divided by 4 times the power required in hover. (The 4 comes from the fact that one can only use half of the kinetic energy stored in the rotor system). Prouty uses a more complex formula that takes in account $C_l$ and $C_d$ of rotor system, but if you use the equation [$\mathrm{Power\ OGE} = (61\cdot10^{-3}/Dia_{rot}) \cdot \sqrt{m^3/ro})$ all in metric (with $ro = 1.225\ \mathrm{kg/m^3}$ at sea level], and divide the value obtained par 0.84 (for TR power, and Transmission losses), and plug this value into the t/k calculation, it works...
So $t/k = \frac{J\cdot\Omega^2}{4\cdot\mathrm{Power\ OGE}}$ in seconds.
The $t/k$ of the Robinson R22 is 0.8 (far too low I agree), and practically, you want $t/k$ around 1.2 to 1.7 sec, so roughly twice the Robinson.
The UltraSport-254 helicopter has an extremely low disk loading and an autorotation descent rate of 900 ft/min. It is said that in autorotation it can land then liftoff and re-land using only the inertia in the rotors.
The Osprey V-22 has an extremely high disk loading. The test data indicate that the aircraft would impacted the ground at a rate of descent of about 3700 ft/min.

Robinson R22 discussion - informative. Comments on auto rotation and much more.

Due to its light weight and low inertia rotor system, the R22 is not forgiving of pilot error or sluggishness. After an engine failure, real or simulated, you and the instructor will have 1.6 seconds to lower the collective and enter an autorotation. Any delay beyond 1.6 seconds will be fatal as the rotor speed, once decayed below 80 percent, cannot be recovered. :-(

The Art of Autorotation
Extremely good auto-rotation tutorial and discussion with a number of pointers to arcane knowledge.

Video - R22 {almost} zero airspeed autorotation landing User comments useful.

R22 pricelist - just for interest

Thank you for your response and especially for the informational links. I also find it interesting and curious that the R22 has a reputation as an unforgiving helicopter, yet many of  flight instruction schools I have researched, seem to offer the R22 as their most affordable model to train in. –

@garyv440 They are low cost (relatively). I've had a total of one flight as an intro to the craft with a few seconds at a time hands n with a hovering instructor watching the dual controls. He did not point out (not too surprisingly) that he had 1.6 seconds to enter autorotate after engine failure or you never can. [!!!] Decades ago we had a deer catching and shooting period of national madness. All who could chase deer with copters did. In one year we lost 30% of our rotary wing base in accidents ! [!!!]. The biggest single type lost was, no surprise , R22s.  With people shooting and live ...

... netting deer from copters and even (really!) jumping onto deer from copters all over our high country losses were inevitable. Deer prices were so good that the odd bullet hole also occurred. Latterly one of the Robinson brothers called to see what the heck we were doing . Conclusion - consistently being flown outside the envelope to maximise  results. Losses are a lot lower these days :-).

Some of the user comments are useful.  Many are not and a lot are wrong.  For example, it is perfectly possible, and safe, to descend vertically.  You only need airspeed at the bottom for the flare to store energy in the rotor for the collective pull to reduce ROD.  it's even possible to descend backwards which feels extremely uncomfortable.  As long as you have flying RPM and forward speed > about 35kts for the flare at the bottom, you're OK.  https://www.youtube.com/watch?v=HlOzln58r2c&list=UUAD3JMNhU880gWPO4rQK5FA&index=1&feature=plcp